The disclosure relates to a field device for controlling a process fluid flow, including a large-volume process fluid stream in a processing plant, such as a refinery, a food processing plant (e.g., a brewery), a petrol chemical plant, or the like.
For leading and controlling of process fluid flows such as large-volume process fluid flows in a processing plant, valves having a large throughflow-cross section, in particular having a connection nominal diameter of at least DN 100, preferably at least DN 200, are employed. As understood by one of ordinary skill in the relevant arts, “DN” refers to the standard designation of pipe size diamètre nominal/nominal diameter (in millimeters).
Valves of this magnitude require large positioning-forces and a high drive- or actuator-power in order to achieve or hold a control position or in order to drive into a safety position. In order to provide the large positioning forces, pneumatic actuators can be employed which are controlled by a pneumatic position controller. Pneumatic actuators are connected to a constant pneumatic pressure source, which can be subjected with a pneumatic control signal via the pneumatic position controller by means of a set position control, in order to move the control valve for adjusting the process fluid stream. The pneumatic actuator has the advantage of an absent or low electric energy consumption so that an explosion hazard in the surrounding of the position controller is significantly reduced, however, the pneumatic circuitry causes relatively high assembly- and maintenance costs.
In order to reduce the energy required by the actuator for adjusting positioning a large valve, part of the process fluid stream can be diverted and can be used as actuator fluid for controlling the large valve. An arrangement of the previously described type, in which an actuator of a primary valve is fed by process fluid, is described in DE 10 2010 037 898 A1. In the known arrangement, a primary valve is provided including an inlet, an outlet and a valve passage arranged between inlet and outlet, which valve passage is closeable by a movable valve member of the primary valve. The valve member further comprises a passage into a balancing- or working-chamber, which enables a fluid communication between the inlet and the working chamber. The working chamber is connected to the outlet of the main or primary valve via a line. Also within the working chamber is arranged a spring that biases the valve member towards the valve passage. Between the working chamber and the outlet, a secondary or auxiliary adjustment device is arranged having a secondary or auxiliary valve and an electric actuator for adjusting the secondary valve. By closing the secondary valve, a pressure can be generated in the working chamber such that the fluid forces and the spring biasing force hold the main valve member in a closed position. When the auxiliary valve is opened, the process fluid can escape from the working chamber towards the outlet and the working chamber pressure sinks. As soon as the spring forces are overcome by the process fluid pressure in the inlet acting on the valve member, the main valve member moves out of the valve passage and opens the main valve.
The known field device is, however, disadvantageous in that, within the auxiliary adjustment device, the process fluid must be led in immediate proximity to the electric actuator power of the auxiliary actuator. Due to the high inflammability of certain process fluids, a relatively large constructive effort is required in order to meet the standards of explosion protection in such control devices. Furthermore, the known arrangement causes high costs during field device installation because the electric actuator energy must be led to the field device, possibly for several hundred meters, of cableway within the processing plant in an explosion-proof manner.